A Senate inquiry into the death of an asylum seeker on Manus Island has heard conflicting evidence from the Immigration Department and the company running the detention centre.

Iranian asylum seeker Reza Barati died in February during a night of riots at the immigration detention centre on Papua New Guinea.

Senators have been told the company in charge of the centre at the time, G4S, had serious concerns about the PNG police mobile squad and had repeatedly asked the department to upgrade fencing and security.

The department has vehemently rejected that it was ignoring those requests or that there was a "go slow" on processing asylum claims.

Senior G4S managers have taken considerable time in the inquiry running through the events at the Manus Island centre in mid-February, when detainees rioted and Barati sustained fatal injuries.

"A hail of missiles of all sorts, from metal poles, pieces of glass, rocks that were the size of my fist and greater," the centre's general manager at the time of the unrest, John McCaffrey, said.

"I remember looking, at one stage, into the night sky and seeing the sky completely filled with missiles. That would ebb and flow through the evening, through the hours that we were there, from times when it was that level of violence to a lesser degree of stones being thrown."

"The accounts that we have actually got just seem to refer to PNG, G4S PNG locals or nationals, they don't name specifically who those PNG people are," he said.

At least one other asylum seeker was shot, and PNG police went into the centre.

G4S emails reveal concern about PNG police

An email from Mr McCaffrey a week before the riot was tabled and shows he warned the Immigration Department's senior officer on Manus Island that if the mobile squad was deployed to a violent situation, it could escalate quickly and could result in very serious injury or death.

Mr McCaffrey's colleague Kevin Pye said G4S did not authorise the PNG police mobile squad to enter the centre on the night of the riot.

"I wasn't convinced at that stage, that less people would die by mobile squad intervention as opposed to more people would die without it," Mr Pye said.

The Senate committee has been told G4S had ongoing concerns about the mobile squad, but that they increased throughout January because the possibility of them being required was increasing as tensions inside the centre grew.

G4S attributes two main factors to the eventual violence: increasing concern from asylum seekers about the apparent lack of progress on their refugee claims, and inadequate security fencing and lighting.

Immigration Department denies 'go slow' on processing claims

During the hearing Immigration Department secretary Martin Bowles rejected a suggestion from Labor Senator Lisa Singh that there had been any direction from the Australian Government about the speed of refugee processing.

Mr Bowles: It does take time, Senator.

Senator Singh: Was there a go-slow direction from the Minister's office?

Mr Bowles: Absolutely not. The department is a professional organisation who goes about its business trying to deliver the best outcome for the people they deal with, Senator.

Mr Bowles also rejected an assertion that the department had ignored requests from G4S for improved fencing.

"We work with G4S to try and get the relevant information we require to legitimately and legally do things within the Australian system," he said.

"We'll continue to do that with any contractor and we'll ask as many questions as we need to to get that arrangement underway. To think, though, that nothing is happening because there are requests is not correct.

"We have been working on this issue for quite a while. As I said, it is quite a complex issue; it's not just as simple as erecting a pool fence."

The managing director of immigration services for G4S Australia, Chris Manning, told Greens Senator Sarah Hanson-Young they did not get formal responses to their requests.

"We never received a response, and I heard the department's officials talking about this," he said.

"Understandably there's a lot of work to be done behind the scenes on these things [but] we never received a response to the October proposal for fencing, we didn't receive a response to the proposal for a logistics hub."

The G4S contract for Manus Island ended in March, and the centre is now run by Transfield.

The inquiry is due to take evidence in Canberra for the rest of this week before delivering its report to the Senate in mid-July.